To collectingly drain oil remaining below a nut part without expanding oil and dirtying surroundings while draining roughly all of oil in a drain structure having the nut part arranged above an oil pan.

In a drain structure of an oil pan having the nut part 7 for making a drain bolt 6 which can block a drain hole 5 detachably threadedly engage with provided on an upper surface 2j of a oil pan bottom wall 2s at a drain hole section, the nut pat 7 consists of a metal plate member 10 including a vertical screw cylinder part 9 positioned on an upper side of the drain hole 5 and having female screw 9a on an inner circumference and a flange part 10 connected to an outer circumference side of a lower end part of the screw cylinder part 9 and formed in a state of getting close to the upper surface 2j of the bottom wall 2s, a small hole 12 for draining remaining oil is formed at a boundary part 11 of the screw cylinder part 9 and a flange part 10, and the small hole 12 is set in an inclined condition in which a lower end part 12a of an inner circumference surface of the hole becomes lower as it goes toward a drain hole center P.